A non-geostationary satellite system comprises a group or constellation of satellites that orbit the earth at altitudes other than geostationary orbit (c.a. 36,000 kilometers above the earth's surface). The non-geostationary satellite systems that are in low earth orbit (LEO) have less propagation loss and less propagation delay than geosynchronous satellite systems due to the lower orbit of the non-geostationary satellites. As a consequence, such satellites are better suited than geostationary satellites for interactive communications, such as internet services.
Geostationary satellite systems have an orbital period equal to the rotation period of the Earth and therefor appear, from Earth, to be at a fixed position in the sky. Non-geostationary satellites move at relatively higher speeds and therefore appear to an earthbound observer to pass overhead from horizon to horizon. Because of this relative movement between non-geostationary satellites and the Earth, such satellites move in and out of range of earth-bound user terminals. Such terminals must therefore switch their communications link from one satellite to the next (i.e., hand-off) to achieve continuous communications.
In some systems, radio transmissions from the satellite to the user terminal are in the form of multiple independent beams aimed in different directions. So, in addition to earth-bound user terminals contending with the hand-off between satellites, there is a hand-off between individual beams of an individual satellite, as the coverage area of a satellite moves past a particular user terminal.
In an optimal situation, each satellite is properly oriented in space such that the beams emanating therefrom are “pointing” in a specified direction. The reality, however, is that there can be an error in the satellite's attitude, in addition to any other errors pertaining to individual beams, such that the beams are not pointing exactly in accordance with the system design. Such inaccuracies in beam pointing lead to a reduction in the signal-to-noise ratio (SNR) at the user terminals.
Beam pointing affects SNR in two ways. One way is that if two neighboring satellites in the constellation point slightly away from each other, a gap in coverage may result on the ground between those satellites. A second way in which SNR can be affected is when a single satellite is pointing slightly the wrong way, and the dividing line between two user beams from that satellite has shifted position (e.g., ahead or behind, etc.) on the Earth's surface. This dividing line is defined as being the location where the signal strengths from both beams are equal. In systems in which the user terminal switches its telecommunications connection from one beam to the next based on time (as calculated a priori from knowledge of the ephemerides and the terminal's position), if the dividing line has shifted from its expected position, the terminal will experience unequal signal strengths before and after the shift.
Currently, there are several ways to deal with this problem. One way is to use the technique common in mobile telephony, wherein the user terminal compares the power of the signal currently being received to the power of other beams received on other channels. As soon as the received power of another beam exceeds that of the currently-received signal, the user terminal performs a switch. At that time, the received signal strength of the two beams will be very close to one another, resulting in little change in SNR. This approach, however, results in a significant amount of overhead traffic. A second way to address the problem is to design each satellite with very tight tolerances (˜0.2 deg max) in beam pointing. But this necessitates relatively more expensive hardware on the satellite in addition to tight manufacturing tolerances. An inexpensive satellite control system will not be likely to meet this tolerance. A third approach is to accept that a relatively poorer quality of service will be provided to the user.
This first approach results in an excessive amount of telecommunications traffic, the second approach attempts to avoid the problem, and the third approach ignores the problem. None of these approaches is particularly satisfactory.